synapse (thing)

This info applies to chemical synapses only. They are the most common type of synapse. Chemical synapses have the advantage of being dynamic. Their properties can be altered over time, whereas electrical synapses are fixed. The altering of synaptic properties is essential to processes like learning and adaptiation. It is not uncommon for several types of neurotransmitters to be released in varying concentrations by a single neuron. It is important to note that a single neuron will release the same transmitters at all of its synapses.

Individual neurons can perform some processing of inputs through dendritic integration, but the true power of neural computation comes from the arrangement of large systems of neurons. Neural systems use a principle called parallel processing. It is an extremely powerful method of computation. The idea is that instead of sending all of your data through one large processor, you divide it up into pieces and send it to many many small processeors (individual neurons in this case). Each of these small processors does a little bit of computation, and sends an output to other small processors. People who can't afford supercomputers but need a lot of power often do similar things with computers (beowulf system). Not only is parallel processing powerful, but it is a way to process information quickly with individual components which are relatively slow. Instead of sending your information a large distance through sequential processors, you send it a short distance through parallel processors. Less distance to travel gives you a faster output. Pretty cool. More on this in a later node.

Now, in order for neurons to do all of this, they have to be able to communicate with one another. This communication is done at a structure called a synapse. It is simply the place where the terminal end of the axon of one cell contacts the dendrites or some other surface on another cell. The neuron transmitting the information is called the pre-synaptic cell, and the neuron receiveing the input is called the post-synaptic cell. At the synapse, there is a (very small) gap between the two cell membranes.

So, how do the neurons get information across this gap? They use chemical signals called neurotransmitters. There are many types of neurotransmitters used in the brain, which allows for a great variety of methods for communication between neurons. At a synapse, neurotransmitter is stored in vesicles inside the presynaptic cell. These vesicles are crowded near the pre-synaptic membrane, and are attached to cytoskeletal filaments, which hold them in place. When an action potential propagates down the axon to the synapse, it triggers the opening of voltage-gated Calcium (Ca2+) channels. This allows calcium to flow into the cell. The calcium then bonds to protiens which are located near the calcium channels. These protiens intiate a molecular event which causes the vesicles containing neurotransmitters to fuse with the synaptic membrane, and release transmitter into the gap between the neurons.

(Calcium is frequently used as a chemical trigger for specific protien actions in the nervous system. This is why there is such a low calcium concentration in neurons at rest. If there was more, it would trigger shit right and left, and the cell would go haywire. Another reason to drink yer milk.)

Once the neurotransmitters are in the gap between the neurons (called the synaptic cleft, by the way) they rapidly diffuse across to the post-synaptic cell membrane. This region of the membrane is highly specalized. It contains numerous ligand-gated ion channels. Calm down. Ligand-gated simply means that the channels are opened and closed by some chemical factor (called a ligand). So, the neurotransmitters bind to the channels on the post-synaptic cell, and the channels open. This allows ions to flow in or out of the neuron, resulting in a membrane voltage change which depends on the particular ion the channels allow to flow, and it's reversal potential. Inputs which increase membrane voltage and, therefore push the neuron closer to action potential generation are called excitatory. Those which lower the voltage and decrease the chances of action potential generation are called inhibitory.

Well, the post-synaptic channels are open, and that's all well and good, but we can't just leave them that way. If we did, the neuron would recieve way too much input, and keep firing or being inhibited until it died. Which is bad. So, there are sometimes proteins in the synaptic cleft which break down neurotransmitters. The neurotransmitters are also reabsorbed by the pre-synaptic neuron, or by glial cells (support cells of the nervous system). Some combination of these actions occurs, which removes active transmitter from the synaptic cleft, and the input ceases, unless the pre-synaptic cell continues to fire.

Now, normally, action potentials are not transmitted one at a time. They are transmitted in rapid groups. This means that transmitter is being rapidly released at the synapse. Some of that is reabsorbed, but some also gets away. So, the neuron needs a method to restock it's transmitter supply. Reabsorption, is one method. The other method is manufacture of neurotransmitter in the soma. There, it is packaged in vesicles and sent down the axon on special cytoskeletal structures, which act as molecular conveyer belts. There is a limit to how fast this can progress though, so it is possible for a neuron which is firing too rapidly to gradually exhaust its supply of ready neurotransmitter. Generally, though, the neuron doesn't fire this rapidly naturally.

Some things to note:
The type of information transmitted at a synapse does not depend on the neurotransmitter released, but on the type of channels contained in the post-synaptic cell, and how they react to the neurotransmitter. Certian neurotransmitters elicit certian reactions in general, but it may not always hold true.
Many neurotoxins work by mimicing neurotransmitters. They can bind to channels and cause them to open, but they are neither reabsorbed or broken down. This results in a constant current flow, which can destroy the neuron.